If three numbers a, b and c form an arithmetic progression, then the middle number is half-sum of the two others:

b = .

Indeed, b = a+d and c = a+2d, then a + c = (a + a+2d) = 2a + 2d.

Hence,  =  = a + d =b.

Therefore, in your case, x = .

In an AP with 3 terms, d or the common difference is calculated as: 2nd term - 1st term = 3rd term  - 2nd term, or 

x + x = n + m

2x = n + m

 

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